1. Field of the Invention
The present invention relates to a CMP head and method of making the same, and more particularly, to a CMP head having diversion openings in the peripheral region and method of making the same.
2. Description of the Prior Art
Chemical mechanical polishing (CMP) is a planarization technique used to planarize the surface of integrated circuits formed on a semiconductor wafer so that high-density multi-layered interconnections can be formed on the planarized surface. Normally, CMP has been applied in the fabrication of inter-layer dielectric (ILD), plug, shallow trench isolation (STI), damascene structure, etc.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a conventional CMP apparatus. As shown in FIG. 1, the CMP apparatus includes a rotatable platen 10, a polish pad 12 bonded to the platen 20 and able to rotate with the platen 10, a slurry supply 14 for supplying slurry 16 to the polish pad 12, and a CMP head 20 used to fix a wafer 18.
During a CMP process, the wafer 18 is placed in between the CMP head 20 and the polish pad 12. The CMP head 20 brings pressure upon the wafer 18 and drives the wafer 18 to rotate so that mechanical polishing effect can be generated between the wafer 18 and the polish pad 12. Meanwhile, the material layer to be planarized of the wafer 18 reacts with the slurry 16, thereby generating chemical polishing effect.
Please refer to FIG. 2 as well as FIG. 1. FIG. 2 illustrates a conventional CMP head. As shown in FIG. 2, the conventional CMP head 20 includes a membrane 22 disposed on a wafer 18, a membrane support 24, a support pad 26 disposed between the membrane 22 and the membrane support 24, and a retaining ring 28 surrounding the membrane 22, the support pad 26, and the membrane support 24. The membrane support 24 includes ventilators 30 and the support pad 26 has corresponding holes 32 so that gas can pass there through.
Please refer to FIG. 3. FIG. 3 illustrates the conventional CMP head 20 during a CMP process. As shown in FIG. 3, gas is implanted into the CMP head 20 through the ventilator 30 of the membrane support 24 and the holes 32 of the support pad 26 during the CMP process. The flexible membrane 22 is pushed by the implanted gas and extends outwardly, thereby bringing pressure upon the wafer 18.
The goal of CMP is to planarize the material layer, but the uniformity of the material layer is critical to the yield of successive processes and the reliability of the devices to be formed. In a CMP process, the pressure that the CMP head 20 exerts upon the wafer 18 is crucial to the uniformity of the material layer.
In conventional CMP design, the ventilators 30 are formed in the central region of the membrane support 24, and the peripheral region does not have any openings. As a result, the collision frequency of gas is higher in certain areas in the peripheral region of the membrane 22, producing higher pressure (as region A shown in FIG. 3). As long as the pressure is unequal, the polishing rate is not equally distributed, and this leads to poor uniformity.
Please refer to FIG. 4. FIG. 4 illustrates a thickness distribution diagram of a material layer after CMP by using a conventional CMP head. In this experiment, the material layer is an oxide layer of 11,000 angstroms disposed on an 8-inch wafer, undergoing 60 seconds of CMP. As shown in FIG. 4, the thickness of the oxide layer in the central region is reduced from 11,000 to 7,300 angstroms, which shows a good uniformity in the central region. However, the thickness of the oxide layer in the peripheral region is evidently thinner (approximately ranging from 70 and 95 mm). This shows the polishing rate is higher in this region, and this over-polishing phenomenon (referred to as fast band effect) occurs to CMP processes frequently.
The fast band effect causes an unfavorable uniformity in the peripheral region, and affects the yield and reliability of the devices to be formed. Therefore, it is an important issue to prevent the occurrence of fast band effect in CMP.